1. Field of the Invention
The present invention relates to an optically sensing device that optically senses physical volume, more particularly, to an optically sensing device such as uses specific sensor materials whose birefringent value varies with external physical volume.
2. Description of Prior Art
Conventionally, an optically sensing device constructed as in FIG. 1 is well-known. Such a device is provided with a sensor material 2 and a quarterwave plate 3 between the polarizers 1 and 4 to allow the volume of outgoing beams to vary according to variations in sensor material 2. Specifically, the volume of outgoing beams varies in accordance with the phase retardation 8 between two polarized beams, which is generated by birefringence of sensor material 2. If no proper birefringence is present in sensor 2, then the phase retardation .theta. starts to vary itself from 0.degree. in response to the variation of external physical volume which starts to vary itself from 0. Since the variation of the beam volume is at its maximum when .theta.=90.degree., in order to securely control the sensitivity at an optimum level, independent of sensor material 2, an element that generates a 90.degree. phase retardation is inserted into the optically sensing device. This is the quarterwave plate 3.
Conventionally, a number of materials having the proper birefringence and specific thickness are suitable for the quarterwave plate 3. Specifically, crystal plate, mica plate, calcite plate, rutile plate, etc., are widely used as high-precision quarterwave plates 3; extended plastic plates are also used for reasons of economy. Nevertheless, these quarterwave plates 3 still face a variety of problems. For example, crystal and mica plates are of monocrystalline structure, and yet, these are not only expensive, but also require extremely delicate processes for obtaining predetermined thickness, thus unavoidably involving expensive processing costs. Similarly, although inexpensive, the quality of extended plastic plates are inconsistent and these are therefore unsuitable where extreme precision is required. Independent of a quarterwave plate that employs birefringence, an element called "Flesnel-rhomb" is well-known. This features the same function as that of the quarterwave plate but is totally different in principle. As shown in FIG. 2, the Flesnel-rhomb employs the phase retardation generated between two polarized beams crossing each other at right angles on total reflection. However, the Flesnel-rhomb 5 requires provision of a specific angle position and an extremely complex value that can only be determined by the refractive index of the medium used, thus, involves very difficult processes. In particular, since the outgoing beams move in parallel with the incident light, "Flesnel-rhomb" was not suitable for practical use. In light of the disadvantage thus mentioned, "Flesnel-rhomb" has rarely been applied to any of optically sensing devices in use today.